1. Field of the Invention
The present invention relates to a metal wire rod for forming coupling elements, which is usually called a Y bar and is applied to a slide fastener, and a metallic coupling element obtained by slicing the same wire rod.
2. Prior Art
In recent years, the slide fastener market has been well on the way to globalization, so that customers tend to purchase products manufactured in a region where the production cost is low. Under this tendency, a struggle for reducing the price of product has been accelerated. Consequently, further quality improvement has been demanded so as to appeal its product value to customers. In order to manufacture the slide fastener at a lower cost and with a higher quality than conventional products, it is necessary to review the rightness of individual components constituting a slide fastener. Particularly, with respect to metallic coupling elements having a slide fastener opening/closing function, the reduction of labor cost and material cost as well as the improvement of mounting strength have been demanded strongly.
Conventionally, three major methods have been adopted for manufacturing this kind of metallic coupling element, which are a press method in which a flat wire rod is produced by pressing, a molding method by die-casting and a Y-bar method in which a metal wire rod having a substantially Y-shaped section is cut to predetermined thickness. According to this Y-bar method, a cut coupling element is composed of a head and a pair of legs extended from the head through a crotch and has a coupling protrusion formed by pressing the head with a punch.
In order to form a metal wire rod having a Y-shaped section for forming the coupling elements, which is used in the Y-bar method, is formed, usually the metal wire rod having a circular section is rolled so as to deform its section into a Y-shape. As compared with metallic coupling elements manufactured by pressing a flat plate, this method ensures a higher strength and wastes no material. Further, because it is suitable for mass production, manufacturing of metallic coupling elements based on the Y-bar method is the most reasonable manufacturing method.
In manufacturing of metallic coupling elements based on the aforementioned Y-bar method, the metal wire rod for forming coupling elements, which has a circular section, is supplied intermittently at a predetermined pitch and is subjected to multi-stage rolling processing so as to produce a substantially Y-shaped section. Particularly an external shape of pairs of right and left leg parts is formed to be opened. The coupling element just after cutting has pawls which are projected inward from a front end of each of the legs.
A wire rod having an irregular shape thus obtained is fed at a pitch corresponding to the thickness of each coupling element. When 1-pitch feeding is completed, the feeding is stopped, so that the wire rod is projected on a cutting die by the thickness of the coupling element at its front end stop position. Here, the projected portion of the wire rod is cut off with a cutting punch and at the same time, the cut coupling element material is carried from the cutting die to a forming die. A forming punch acts on a head of the coupling element material mounted on the forming die thereby forming a coupling protrusion on the head, which is so-called hill-forming.
On the other hand, on the side of the legs of the coupling element material subjected to the hill-forming as described above, a fastener tape is fed, with a predetermined gap relative to the coupling element material, at each coupling element mounting pitch such that an edge portion of the fastener tape for mounting the coupling elements oppose the crotch between the both legs. After the hill-forming is completed, the coupling element is carried toward the coupling-element-mounting edge portion of the fastener tape, so that the coupling element-mounting edge portion is nipped by the pair of legs opened outward. Here, a caulking punch is actuated so as to caulk the pair of legs inward. Thus, implantation of the coupling element to the fastener tape is completed.
Coupling element materials obtained by cutting the aforementioned metal wire rod having a Y-shaped section for forming coupling elements have been disclosed in, for example, Japanese Utility Model Application Laid-Open No. 51-13903, Japanese Patent Application Laid-Open Nos. 6-217810 and 8-56714, and Korean Patent Laid-Open Publication No. 20-229751. According to these publications, an inner face of the crotch of the coupling element is formed in a circular or a flat plane, and an inner face of each of right and left legs opened symmetrically subsequent to the crotch is formed in a mere curved face or a flat plane. This is because it is intended that the entire legs should be bent uniformly when the legs are caulked.
As indicated in the aforementioned Japanese Patent Application Laid-Open No. 8-56714 and Korean Patent Laid-Open Publication No. 20-229751, the opening angle of the right and left legs are necessarily extremely large. This is because the mounting strength of the coupling element to the fastener tape depends on biting strength of the pawls projected inward from front ends of the right and left legs, and the biting strength is mainly based on the projection length of the pawls and the intersection angle of the pawls relative to the extending direction of the legs. Therefore, the opening angle between the right and left legs needs to be extremely large.
Usually, rolling process to be applied to the metal wire rod in multiple stages is carried out by plural rolling rollers disposed so as to surround a wire rod. After this rolling process is completed, the respective rolling rollers move in a direction away from the center of the wire rod. If the pawls are projected beyond an end face of the rolling roller, the end face of the rolling roller interferes with the pawls when the rolling roller leaves, thereby disabling the rolling roller from leaving. To avoid such interference, the inner faces of the pawls need to be so designed as to be parallel to the end face of the rolling roll. As a result, the right and left legs are necessarily opened to a large extent. Additionally, the opening angle of the conventional legs is about 92°.
Further, in terms of strength of the conventional coupling element material, the strength of all portions including its head and legs is substantially equal because the metal wire rod is formed by changing its external shape and sectional shape through multi-stage rolling process.
When the inner faces of the crotch and the legs of the metallic coupling element are mere flat planes or curved faces as described above, uniform positioning accuracy can not be obtained when the same metal wire rod is deformed by rolling. Therefore, it is difficult to process the entire shape of the coupling element symmetrically with respect to a center line connecting a center of the head with a middle point of a straight line connecting front ends of the pair of legs. Further, when the coupling element having such a configuration is mounted on a fastener tape by caulking the legs thereof from outside with a caulking punch, the legs cannot be bent equally from the crotch thereof. Additionally, the coupling element is likely to rotate with respect to the head, so that the coupling element often cannot be mounted accurately on a symmetrical position nipping a coupling-element-mounting portion of the fastener tape.
Particularly, if the opening angle of the right and left legs extending from the crotch is as large as that of the conventional metallic coupling element, the mounting posture of the coupling element on a fastener tape is unstable, so that the coupling element becomes likely to rotate around the head thereof. This causes a fall of productivity in terms of yield rate. Therefore, it is preferable that the opening angle is as small as possible.
On the other hand, from a viewpoint of the mounting strength of the coupling element to the fastener tape, the head of the coupling element is not directly related to the mounting strength, while the right and left legs concern directly the mounting strength and the mounting strength depends on the configuration and strength of the legs. Therefore, it is the head of the coupling element from which the amount of material can be reduced. However, if part of the head of the coupling element is reduced, when it is mounted on the fastener tape by caulking the legs, the strength of the crotch between the head and the legs drops largely because the width dimension of the crotch is reduced due to deformation of the legs.
The present invention has been accomplished to solve the above-described problems. An object of the invention is to provide a metal wire rod, which enables reduction of the material, securing a necessary mounting strength to a fastener tape and strength of an entire coupling element, as well as manufacturing of a coupling element having such configuration and mounting strength as to stabilize a mounting posture of the coupling element to the fastener tape. A further object of the invention is to provide a coupling element obtained from the same wire rod.